1. Field of the Invention
This invention relates to motor controllers, and more particularly to a motor controller adopting a winding changeover scheme.
2. Description of the Related Art
In the existing vehicular alternating-current motors, it is a practice to carry out a changeover control of the stator winding from a star connection (Y connection) to a triangular connection (Δ connection) in order to improve the vehicle performance in medium and high speed ranges. There is a disclosure, for example, in JP-A-61-73591 (FIGS. 1-3 and related explanations thereof), as a motor control method based on such a winding changeover scheme. Namely, when reaching a speed higher than the upper limit of a speed range the ratio of an inverter output voltage and an output frequency is kept constant, the stator windings are changed over from a star connection (Y connection) into a triangular connection (Δ connection), thereby increasing the phase voltages to a √3 times those of before changeover and the phase currents to a 1/√3 times those of before changeover. By doing so, high output is maintained even when there is an increase of rotation rate.
Meanwhile, JP-A-6-217596 (FIGS. 1-5 and related explanations thereof) describes that, in an electric-automotive motor having a double motor structure for driving the output shaft, the connection between motor windings is changed from a star connection (Y connection) to a triangular connection (Δ connection) in order to extend its rotation-rate range, thereby obtaining a desired output torque even in a high speed range. The power to the motor is supplied through contactors after converting a direct-current power source (battery) into three-phase alternating current power by use of two inverters. The contactors receive a winding-changeover signal from the ECU (electronic control unit) and change the connection between the windings of the motor.
However, such connection changeover with using the contactors is, concretely, by a configuration as in FIG. 3 or 5 in JP-A-6-217596, i.e., for one inverter route, each contactor is connected to the motor winding through six lines and six contacts. There is a problem that the contactor for use in turning on-off a great current is expensive and moreover to be shortened in life by frequent on-off uses. Meanwhile, the contactors can be configured by power semiconductor devices. However, in such a case, there requires an increased number of power semiconductor devices resulting in a problem of size and cost increase for the device.